LED Grow Lights Decrease Internodal Spacing

Abstract

Nine Super Sweet 100 tomato plants (Lycopersicon lycopersicum) were started from seed and grown for 37 days with controlled nutrient and H2O input, ambient CO2 temperature, and humidity under three different light sources (Hortilux 400-watt HID bulb, 420-watt Black Dog LED flowering model [BD450F], and 270-watt Black Dog LED vegetative prototype [270W]) in order to compare the effects of each light source on internodal distance and nodal density, and therefore on plant health and potential fruit yield. Single Factor ANOVA analysis allowed us to conclude that there was a significant difference in both the average internodal distance and the average nodal density between the plants grown under the different light sources; the three plants grown under the BD450F had the lowest average internodal distance (3.18 cm) and the highest average nodal density (0.31 nodes/cm). These results are indicative of the effects of spectrum quality on plant growth, and have major implications regarding the future of Black Dog LED plant grow lights in the indoor horticulture industry.

Introduction

Light Emitting Diodes (LEDs), commonly used as indicator lights and in digital displays, are valued in the lighting industry for a number of reasons: Solid-state construction means there is no filament to burn out, which greatly increases life span; directional technology allows the light to be focused where it is needed without bulky reflectors; diodes constructed without toxic mercury allow for more environmentally friendly disposal; and the production of more light per watt creates a more efficient light source (Harris & Fenton, 2012; Sewell, 2011). Recently, LEDs have become popular as a light source for growing plants. In addition to the reasons listed above, this is because LEDs can be made to emit almost any wavelength of light. Individual diodes can therefore be arranged in specific combinations of wavelengths to create a panel with a custom spectral output. Unfortunately, there is very little existing research comparing modern LEDs to more traditional plant grow lighting sources (such as high pressure sodium [HPS] and metal halide [MH] bulbs); of the research that does exist, most utilizes outdated LED light panels with incomplete spectrums (Schuerger, Brown, and Stryjewski, 1997). This is unfortunate because LED lighting technology has grown in leaps and bounds over the past few years, making LED panels true contenders in the indoor plant grow lighting industry.

The most commonly used light sources for growing plants indoors are high intensity discharge (HID) bulbs utilizing either MH or HPS bulbs to achieve the desired spectrum. MH bulbs radiate primarily blue light with minor amounts of red and ultra-violet light, while HPS bulbs radiate primarily red and infrared light with minor amounts of blue light. Many LED grow lights on the market today radiate some combination of red, blue, ultraviolet, and/or infrared light. Researchers at Black Dog Laboratories have determined that while these four wavelengths of light are very important in plant growth, they are not the only wavelengths used by plants for photosynthesis (Black Dog LED, 2012). All Black Dog LED light panels include the proprietary Phyto-Genesis Spectrum™, which utilizes the variety of LED wavelengths available to create a combination of up to 17 different wavelengths of light, each proven useful to growing and flowering plants (Black Dog LED, 2012). These full-spectrum, high-intensity lights are designed to replace traditional indoor plant grow lights, using a more complete spectrum to produce equal or better results with less energy.

The purpose of this study is to demonstrate that state-of-the-art, high-intensity, and full-spectrum Black Dog LED plant grow lights will produce healthier, more robust plants than traditional HID grow lighting when nutrient, energy, and environmental inputs are controlled.

Materials and Method

On 10 June 2012, approximately 20 Lake Valley Seed brand Super Sweet 100 Tomato seeds were sealed in a Ziploc bag with a damp paper towel and left to germinate. On 17 June, the germinated seeds were distributed equally among nine 5-inch plastic pots in a mixture of Roots Organic 707 soil and Mycos and watered with tap water at 40ppm. The pots were each assigned a number (1–9) and placed in groups of three in each of three adjacent, three-sided booths. Each booth measured approximately 93.0 cm x 89.5 cm x 225.0 cm, and was completely lined with black 6 millimeter and white poly "Panda Tarp" with the white side facing inward to maximize reflectance. Each booth was set up with an individual light source: Booth 1 (pots 1, 2, 3) contained a 400-watt Hortilux MH bulb in a Hydrofarm Daystar hood with a Nextgen 600w/400w Digital ballast (HID). Booth 2 (pots 4, 5, 6) contained a 420-watt Black Dog LED Flower Spectrum panel (BD450F). Booth 3 (pots 7, 8, 9) contained a 270-watt Black Dog LED Vegetative Spectrum prototype panel (270W). On 28 June, the seedling plants were thinned out to leave nine plants of the most similar heights, with one in each pot.

Each light was placed at an initial height of 3.0 feet above the ground, or approximately 2.5 feet above the rim of each pot. The HID light was set up with a glass panel and 4-inch high-output can fan in order to avoid the heat stress observed in previous experiments. Each light was connected to a timer and run on a cycle of 18 hours on and 6 hours off. A Jarden Consumer-Fans Westpointe Twin Window Fan was placed as an exhaust in the only window in the room (closest to Booth 1). Two humidifiers (Walgreens Model 500379 and Crane Ultrasonic Humidifier) were placed approximately four feet in front of Booth 2 (the most equidistant location), six feet high on the wall. The humidifiers were filled twice a day for the duration of the experiment. No additional venting, heating, or cooling methods were employed anywhere in the room. There was no CO2 input, so it is assumed that each booth was receiving equal amounts of the gas.

On day 1 of the experiment, each booth was sprayed down with a 3.5% hydrogen peroxide solution (the solution did not come into contact with any of the plants), and each plant was watered with a solution of House and Garden nutrients according to the week 1 Vegetative Feeding Schedule: 16.0 mL Soil A/B, 0.8 mL Drip Clean, 2.2 mL Roots Excelurator, 1.6 mL Amino Treatment, 7.6 mL MultiZen, 3 mL Algen Extract, and 2.0 mL Nitrogen Boost in 2.0 gallons of water. On day 7, each plant was foliar-fed with a solution of Magic Green Foliar Spray Fertilizer in a solution of 2.0 mL per quart, as well as a solution of House and Garden nutrients according to the week 2 Vegetative Feeding Schedule: 19.0 mL Soil A/B, 0.8 mL Drip Clean, 2.2 mL Roots Excelurator, 2.0 mL Amino Treatment, 7.6 mL MultiZen, 3.0 mL Algen Extract, and 2.0 mL Nitrogen Boost in 2 gallons of water. It was observed on day 7 that the plants under both of the LED lights required more water than the three under the HID light. On day 8 of the experiment, each of the nine plants was transplanted to 8.5 inch x 9 inch pots, watered with 1 oz of Xtreme Juice, and all of the flower sites were removed. On day 14 of the experiment, the plants received a solution of House and Garden nutrients according to the week 3 Vegetative Feeding Schedule for tall vegetative crops: 19.0 mL Soil A/B, 0.8 mL Drip Clean, 2.2 mL Roots Excelurator, 2.8 mL Amino Treatment, 7.6 mL MultiZen, 3.0 mL Algen Extract, and 3.0 mL Nitrogen Boost in 2 gallons of water. The plants were each administered a solution of Magic Green Foliar Spray Fertilizer in a solution of 2.0 mL per quart. On day 19, each plant was watered with a 1-oz solution of Xtreme Juice. On day 21, each plant was watered with tap water. On day 32, the plants all received a solution of House and Garden nutrients according to the week 4 Vegetative Feeding Schedule for tall vegetative crops: 21.0 mL Soil A/B, 0.8 mL Drip Clean, 2.2 mL Roots Excelurator, 4.0 mL Amino Treatment, 7.6 mL MultiZen, 3.0 mL Algen Extract, and 3.0 mL Nitrogen Boost in 2 gallons of water. On days 1, 7, 19, and 37 of the experiment, the height, number of nodes, and distance between nodes was recorded for each plant.

The raw data was entered into Excel, and two Single Factor ANOVA tests were run: one to determine whether there was a significant difference between the average internodal distance of the plants under the different light sources, and another to determine whether there was a significant difference between the average nodal density of the plants under the different light sources.

Results

Internodal Distance

In order to ensure that there was no significant difference in mean internodal distance between the plants before starting the experiment and therefore that no plant had an advantage over the others, the internodal distances were measured for each plant before they were placed in their treatment booths, and this data was analyzed using a Single Factor ANOVA test. Table 1 shows the results of this test.

The calculated F-value of 2.12 is less than the critical F-value (α = 0.05, df = 2) of 3.16, meaning that we fail to reject the null hypothesis that there is no significant difference in internodal distance between the plants before the start of the experiment.

The internodal distances recorded on the final day of the experiment were organized into Excel. Table 2 shows a summary of the raw data.

The calculated F-value of 13.28 is greater than the critical F-value (α = 0.05, df = 2) of 3.05, meaning that we reject the null hypothesis and can conclude with 95% confidence that the difference in internodal distance of the plants under the three different light sources is greater than can be attributed to chance alone.

Internodal Density

In order to ensure that there was no significant difference in mean internodal density between the plants before starting the experiment, and therefore that no plants had an advantage over the others, the average nodal density (number of nodes divided by height in cm) was calculated for each of the plants before they were placed in their treatment booths, and this data was analyzed using a Single Factor ANOVA test. Table 4 shows the results of this test.

The calculated F-value of 0.43 is less than the critical F-value (α = 0.05, df = 2) of 5.14, meaning that we fail to reject the null hypothesis that there is no significant difference in nodal density between the plants before the start of the experiment.

The height and number of nodes recorded for each plant on the final day of the experiment were used to calculate nodal density for each plant. This data was then organized into Excel. Table 5 shows a summary of the raw data.

The calculated F-value of 5.65 is greater than the critical F-value (α = 0.05, df = 2) of 5.14, meaning that we reject the null hypothesis and can conclude with 95% confidence that the difference in nodal density of the plants under the three different light sources is greater than can be attributed to chance alone.

Discussion and Conclusion

The purpose of this study was to demonstrate that high-intensity, full-spectrum Black Dog LED plant grow lights will produce healthier, more robust plants than traditional HID grow lighting when nutrient, energy, and environmental inputs are controlled.

The two factors used as indicators of plant health and quality in this experiment were internodal distance and nodal density. These factors were chosen because low internodal distance and high nodal density are signs that a plant is not wasting large amounts of energy on vertical growth alone. In fruiting plants, more vertical growth is far from ideal and is often referred to in the pejorative sense as "stretching". Stretching occurs when a plant is not receiving the complete spectrum required for photosynthesis (specifically, there is not an adequate ratio of red to far-red light, nor is there an adequate amount of blue light) or when the ambient temperature is too high and is undesirable for a number of reasons (Rajapakse & Kelly, 1992; Appelgren, 1991; Frimanslund & Grimstad, 1993). Stretching generally predicates a reduced fruit yield, results in weak stems that need to be supported, and greatly reduces light penetration to the lower portion of the plant. Internodal distance and nodal density both relate to the number of nodes along the main stem of the plant, and the number of nodes positively correlates with the number of fruiting sites.

When comparing internodal distance and nodal density between the plants under the different light sources, statistical analysis allows us to reject the null hypothesis in both cases and accept the hypotheses that different light sources resulted in different internodal distances and different nodal densities. According to Table 2, the lowest mean internodal distance occurred in the plants grown under the BD450F by a margin of almost 2.0 cm. According to Table 5, the greatest average nodal density occurred in the plants grown under the BD450F by a margin of 0.11 nodes per cm. Based on this information, we can conclude that the 420-watt Black Dog flowering model grow light produces plants with the lowest average internodal distance and highest average nodal density vs a 400-watt HID light. It can further be extrapolated, using internodal distance and nodal density as indicators of plant health and fruit production, that plants grown under the BD450F will be healthier and yield more fruit than plants grown under HID.

The results of this study have proven with statistical significance that light quality does indeed have an impact on plant health, and that the power and spectrum contained within Black Dog LED grow panels will produce healthier plants than traditional indoor grow lighting sources. Previous research has shown that the quality of the spectrum put out by a light, as well as ambient temperature, determines whether the plant will put more energy into vertical stretching or node development and lateral branching, and the results of this experiment support that evidence. The 15-band spectrum contained within the 420-watt Black Dog LED grow light panel outperformed the 400-watt HID bulb in terms of plant health and potential fruiting sights with no extra inputs of energy, water, or nutrients. Also worth noting is that the BD450F, with an actual wattage of 420 watts, outperformed the HID setup, with a total wattage of 528 watts. A 400-watt ballast requires roughly 455 watts to run plus the additional input of energy required for a blower fan of 73 watts which was necessary to avoid heat stress. These findings have major implications for small or large indoor grow operations when it is important to consider heat stress, space limitations, energy inputs and consumption, and of course, quality of fruit yield. While more research with larger population sizes and different plant species would paint an even better picture of the difference in quality between plants grown under high-intensity, full-spectrum LED lights versus more traditional lighting sources, it is clear that Black Dog LED grow lights are a viable option for low-energy, efficient, and effective sources of lights for serious indoor growers.